In European Patent No. 0548159 (and the corresponding U.S. Pat. No. 5,316,215), there is claimed a liquid delivery apparatus comprising
(i) a liquid reservoir into which a liquid may be introduced via an inlet to pressurise said liquid in the reservoir;
(ii) an outlet via which said liquid may be discharged from the reservoir under the pressure of the liquid in the reservoir;
(iii) a valve between the reservoir and the outlet to control passage of liquid from the reservoir to the outlet, said valve having (a) an opening, (b) a closure member adapted to close the opening, and (c) a biasing means, the arrangement of the components of the valve being such that the valve is normally held closed under the force of the biasing means and the pressure of the liquid in the reservoir;
(iv) a valve control mechanism for controlling the operation of the valve in response to the pressure of the liquid in the reservoir; and
(v) a means for transmitting the pressure in the reservoir to the valve control mechanism;
wherein the valve control mechanism comprises a moveable element which is capable of being acted on by the pressure of the liquid in the reservoir and transmitting a resultant force to the closure member of the valve in a direction to open the valve, and wherein the respective effective surface areas of the moveable element and the closure member and the force of the biasing means are chosen such that the closure member is openable when the pressure of the liquid in the reservoir reaches a predetermined level.
The liquid delivery apparatus disclosed in the aforesaid patents includes a housing and an inner moveable wall which sub-divides the housing into first and second chambers which are of variable volume depending upon the position of the inner moveable wall in the housing, the first chamber containing air as a compressible fluid and the second chamber defining the liquid reservoir, and the inner moveable wall being a flexible membrane such as a flexible bag.